THE HEMISPHERES. 473 



the whole geographical division of the hemisheres. It commences as a 

 transverse indentation on the under surface of the brain, running thence 

 outward, backward, and upward, thus forming the anterior boundary 

 of the middle or temporal lobe. In the inferior animals, the whole 

 hemisphere is seen to be curved round this fissure, the convolutions 

 generally following its course and bending round its upper extremity in 

 an arched form ; and in the human brain this arrangement of the lateral 

 convolutions is also distinctly visible. 



On the outer side of the cerebral hemisphere, where it emerges from 

 the base of the brain, the fissure of Sylvius presents, in man, two dis- 

 tinct branches, namely a shorter, anterior, ascending branch (a), and a 

 longer, posterior, more horizontal branch (6, 6, 6). At its middle and 

 anterior portions, this fissure is very deep, and conceals within its folds 

 a projecting group of short radiating convolutions, belonging to the 

 under surface of the brain, between the anterior and temporal lobes, 

 called the "Island of Reil." 



The second fissure in importance, visible upon the convexity of the 

 hemisphere, is the Fissure of Rolando (II). This fissure runs from 

 near the median line transversely outward and somewhat obliquely for- 

 ward, reaching nearly to the middle of the fissure of Sylvius, and form- 

 ing the boundary line between the frontal and parietal portions of the 

 hemisphere. It is bordered b}^ two convolutions, one on each side, 

 running parallel w T ith itself, namely, the anterior and posterior central 

 convolutions (4, 5). 



The third principal fissure is the Parietal Fissure (P). It starts 

 from immediately behind the posterior central convolution, and runs 

 backward through the parietal portion of the hemisphere, curving some- 

 what downward upon itself toward its posterior extremity. Outside 

 and below it are the arched convolutions about the fissure of Sylvius; 

 inside and above it is a convolution running parallel with the great 

 longitudinal fissure. 



Beside the three fissures just named there are five others, which, though 

 less deep and strongly marked, are constantly present and show a con- 

 siderable regularity in their position and arrangement. Three of them 

 are in the anterior or frontal lobe, in front of the fissure of Rolando. 

 The first runs parallel with the fissure of Rolando, and a little in 

 advance of it, toward the anterior extremity of the hemisphere. It is 

 called the "prsecentral fissure." The second runs through nearly the 

 whole length of the frontal lobe, in a general direction parallel with 

 that of the great longitudinal fissure. It divides the upper from the 

 middle portion of the frontal lobe, and is called the " superior frontal 

 fissure." The third is the "inferior frontal fissure," and surrounds the 

 upper end of the short ascending branch of the fissure of Sylvius. The 

 two remaining fissures of this grade, visible in a lateral view of the 

 brain, are situated in the temporal lobe, below and behind the fissure of 

 Sylvius, with which they run in a general parallel direction. 



In addition to the fissures already described there are many others 

 31 



